Technical Field
Embodiments of the invention relate generally to power generation and, more particularly, to a system, method and apparatus for controlling the flow direction, flow rate, and temperature of solids utilized in a power generation process.
Discussion of Art
Fluidized bed combustion (FBC) is a combustion technology used in power plants, primarily to burn solid fuels. FBC power plants are more flexible than conventional power plants in that they can be fired on coal, coal waste or biomass, among other fuels. The term FBC covers a range of fluidized bed processes, including circulating fluidized bed (CFB) boilers, bubbling fluidized bed (BFB) boilers and other variations thereof. In an FBC power plant, fluidized beds suspend solid fuels on upward-blowing jets of gas during the combustion or chemical reaction process in a combustor, causing a mixing of gas and solids. The fluidizing action, much like a bubbling fluid, provides a means for effective chemical reactions and heat transfer in the combustor.
During the combustion process of fuels which have a sulfur-containing constituent, e.g., coal, sulfur is oxidized to form primarily gaseous SO2. In particular, FBC reduces the amount of sulfur emitted in the form of SO2 by a desulfurization process. A suitable sorbent, such as limestone containing CaCO3, for example, is used to absorb SO2 from flue gas during the combustion process. In order to promote both combustion of the fuel and the capture of sulfur, FBC power plants operate at temperatures lower than conventional combustion plants. Specifically, FBC power plants typically operate in a range between about 850° C. and about 900° C. Since this allows coal to combust at cooler temperatures, NOx production during combustion is lower than in other coal combustion processes.
To further increase utilization of the fuel and efficiency of sulfur capture, a cyclone separator is typically used to separate solids, e.g., unutilized fuel and/or limestone, entrained in flue gas leaving the combustor. The separated solids are then returned to the combustor via a recirculation means, e.g., a recirculation pipe, to be used again in the combustion process. A sealpot, sometimes referred to as a “j-leg,” maintains a seal between the combustor and the cyclone separator to prevent unwanted escape of flue gas from the combustor backward, e.g., in a direction opposite to flow of the separated solids into the combustor, through the recirculation pipe.
In connection with the above, air systems in an FBC power plant are designed to perform many functions. For example, air is used to fluidize the bed solids consisting of fuel, fuel ash and sorbent, and to sufficiently mix the bed solids with air to promote combustion, heat transfer and reduce emissions (e.g., SO2, CO, NOx and N2O). In order to accomplish these functions, the air system is configured to inject air, designated primary air (PA) or secondary air (SA), at various locations and at specific velocities and quantities.
In addition, fluidizing air or gas and transport air or gas are typically supplied to the sealpot to facilitate the flow of solids and gas through the sealpot, as disclosed in U.S. Pat. No. 9,163,830, which is hereby incorporated by reference herein in its entirety. In particular, as is known in the art, solids from the chemical process that move downward through a feedpipe into the sealpot from the cyclone separator are fluidized by the fluidizing air or gas. The fluidized solids are then transported to a discharge pipe by the fluidizing and/or transport air or gas and ultimately back to the combustor. Thus, the sealpot forms a seal, thereby preventing flue gases in the combustor from flowing backward through the sealpot, e.g., upward through the feedpipe back into the cyclone, as is known in the art.
More recently, sealpots have also found use in chemical looping systems. Chemical looping systems utilize a high temperature process, whereby solids such as calcium or metal-based compounds, for example, are “looped” between a first reactor, called an oxidizer, and a second reactor, called a reducer. In the oxidizer, oxygen from air injected into the oxidizer is captured by the solids in an oxidation reaction. The captured oxygen is then carried by the oxidized solids to the reducer to be used for combustion and/or gasification of a fuel such as coal. After a reduction reaction in the reducer, the solid products with some un-reacted solids are returned to the oxidizer to be oxidized again, and the cycle repeats. In such systems, a sealpot may be utilized to prevent a pressure differential that could cause backflow, as discussed above. For example, a sealpot may be utilized in between the output of the oxidizer and the input of the reducer to provide a flow of oxidized solids to the reducer and prevent backflow therefrom.
In both types of systems, the flow rate and temperature of the solids entering the combustor/reducer (e.g., coal in a traditional FBC system, and limestone/calcium oxide, or metal oxide, in a system that incorporates chemical looping) are important parameters that affect chemical reactions. In particular, the temperature of the circulating solids must often be reduced prior to entering the reactor in order to ensure a desired level of chemical reaction.
In view of the above, while the design of existing sealpots is generally suitable for controlling a flow of solids along a single pathway and preventing backflow, control of the flow rate and the temperature of such solids, as well as control of the flow of solids along multiple paths, remains challenging and inefficient. Accordingly, there is a need for an integrated system and apparatus that provides for more precise and flexible control of the flow direction, flow rate, and temperature of solids.